If you are trying to copy on the fly then 16x would probably be your limit. Copying from your hard drive should allow maximum burn speed.
Just make sure the didks you are using are actually recognised as higher speed by the burner. Some 40x, 48x and 52x disks are seen as much slower speeds. I have some 8x CDRs that are recognised as 24x on my DVD-106D, and some 52x that are only recognised as 16x. It's just a matter of finding compatible media.
Keep your firmware up to date. That can help.